The development of reliable, affordable and efficient sensors is a key step in providing tools for efficient monitoring of critical environmental parameters. This review focuses on the use of tapered optical fibres as an environmental sensing platform. Tapered fibres allow access to the evanescent wave of the propagating mode, which can be exploited to facilitate chemical sensing by spectroscopic evaluation of the medium surrounding the optical fibre, by measurement of the refractive index of the medium, or by coupling to other waveguides formed of chemically sensitive materials. In addition, the reduced diameter of the tapered section of the optical fibre can offer benefits when measuring physical parameters such as strain and temperature. A review of the basic sensing platforms implemented using tapered optical fibres and their application for development of fibre-optic physical, chemical and bio-sensors is presented.

Quartz crystal microbalance (QCM) sensor array was developed for multi-purpose human respiration assessment. The sensor system was designed to provide feedback for human respiration. Thorough optimization of measurement conditions: air flow, temperature in the QCM chamber, frequency measurement rate, and electrode position regarding to the gas flow—was performed. As shown, acquisition of respiratory parameters (rate and respiratory pattern) could be achieved even with a single electrode used in the system. The prototype system contains eight available QCM channels that can be potentially used for selective responses to certain breath chemicals. At present, the prototype machine is ready for the assessment of respiratory functions in larger populations in order to gain statistical validation. To the best of our knowledge, the developed prototype is the only respiratory assessment system based on surface modified QCM sensors.

Ammonia (NH3) and carbon dioxide (CO2) are important breath biomarkers for diagnosis of renal and respiratory diseases. Simultaneous sensing of the two gases is useful for assessment of Helicobacter Pylori infection. This study introduces a multi-parameter optical fibre sensor by coating two different dyes, thymol blue and tetraphenylporphyrin tetrasulfonic acid hydrate (TPPS) into one optical fibre sensing system for multi-sensing of the two gases. The dyes are separately coated on distal ends of a 2×2 optical fibre coupler using the sol-gel technique. The reflection spectrum contains distinctive absorption peaks of the two dyes in the wavelength domain and hence can be analysed using a single spectrometer. The absorbance at a wavelength of 608 nm corresponding to the absorption peak of the thymol blue exhibits an exponential decrease as the CO2 concentration increases from 0 ppm to 60,000 ppm. The absorption bands at approximately 490 nm and 711 nm corresponding to the Soret band and Q-band of TPPS, respectively, show changes both in absorbance and central wavelengths in response to NH3 concentration in the range from 0 ppm to 80 ppm. The limit of detection and response time for CO2 and NH3 are 637 ppm and 0.15 ppm, 86 s and 83 s, respectively. The proposed sensor demonstrates the capability of sensing NH3 and CO2 simultaneously in a complex gas atmosphere containing other chemicals such as methanol and acetone which are representative of industrial or metabolic compounds that may confound measurements.

Tapered optical fibers with nano-assembled coatings of thicknesses of order tens of nanometres were used for the detection of ammonia gas. The film coating was composed of alternate layers of tetrakis-(4-sulfophenyl) porphine (TSPP) and poly(allylamine hydrochloride) (PAH), which were deposited using the electrostatic self-assembly process (ESA). Exposure of a PAH/TSPP nano-assembled non-adiabatic tapered optical fiber with a waist diameter of 10 µm to ammonia induced significant optical changes in the transmission spectrum of the optical fiber. The fiber optic sensor showed a linear sensitivity to the concentration of ammonia in the range of 10–100 ppm, with response and recovery times less than 100 and 240 sec, respectively. The 3σ limit of detection (LOD) was estimated to be ca. 2 ppm.